The present invention relates generally to a method and system for preventing any optical surges from occurring in a rare earth-doped fiber circuit such as an erbium-doped fiber amplifier (EDFA) etc.
There are known optical amplifiers comprising rare earth (e.g., erbium) element-doped optical fibers. In such an optical amplifier, rare earth elements in an optical amplifier medium or a rare earth-doped fiber section are excited by a pump light, and a signal light coming into the rare earth-doped fiber section causes induced emission by the excited rare earth element, providing amplification of the signal light. These kind of amplifiers are applied to optical booster amplifiers, optical pre-amplifiers, optical repeaters, etc. in an optical transmission system.
FIGS. 1A and 1B are diagrams showing exemplary arrangements of conventional rare earth-doped fiber circuits. In FIG. 1A, the circuit comprises a rare earth-doped fiber amplifier (e.g., an erbium-doped fiber amplifier) 10a and, preferably, an optical isolator 5 the output of which is connected to the output of the amplifier 10a. The amplifier 10a comprises an amplifying medium (or an erbium-doped fiber section) 1, a wavelength division multiplexer (WDM) 3 and a pump light source 2 using a laser diode. A pump light from the pump light source 2 is introduced into the doped fiber section 1 via the WDM 3. The pump light excites the erbium within the doped fiber section 1. A signal light is amplified through induced emission by the excited erbium. The optical isolator 5 may be disposed along output signal path and used to remove any counter-propagating signals from entering doped fiber section 1.
The pump light is introduced from the opposite side of the dope fiber section 1 with respect to the message signal input side of the amplifier 10a, whereas in an EDFA 10b in FIG. 1B, the pump light is introduced from the message signal input side of the amplifier 10b via WDM 4. A rare earth-doped fiber circuit in FIG. 1B preferably comprises an isolator 5 having its input connected to the output of the EDFA 10b and a band pass filter 6 for letting only the message signal pass preventing the pump light component from passing.
In case of an erbium-doped fiber amplifier, the wave length of the signal light is on the order of 1.55 .mu.m and the wave length of the pump light is on the order of 0.98 .mu.m or 1.48 .mu.m.
In a doped fiber amplifier, entering of a signal light into the doped fiber section 1 keeps in an equilibrium the energy accumulated by the pump light and the energy used for amplification of the signal light through induced emission. However, if the signal light has not been entered for more than a certain period of time, then far more energy is accumulated as compared with the equilibrium state in the doped fiber section 1 because the accumulated energy is not used for amplification.
If a signal starts entering the doped fiber section 1 in such a no signal state, a large amount of accumulated energy is used for induced emission at a time, that is, a large power signal light is output from the doped fiber section 1 till the accumulated energy and the energy used for induced emission come to an equilibrium. The large power signal light, which has a pulse shape of a high energy, is called a light surge, and may cause a damage in optical elements constituting the optical amplifier and devices connected along the signal output line.
A technique for coping with the problem of optical surges is well known in the art in which optical surges are suppressed by lengthening the time taken for the signal light to rise sufficiently longer than the easing time constant of the doped fiber amplifier 1. However, the technique fails in complete suppression of optical surge pulses as well as causes a reserve signal source if any (as in a redundant system) to take much time for starting.
Japanese Patent unexamined publication No. hei5-63653 (1993) discloses a technique for controlling the output power through monitoring the output power. However, the technique is not effective in preventing optical surges to occur transitionally in case when a signal light enters a doped fiber section in the no signal state or in the case when the signal light has recovered after a temporary failure as may happen in a redundancy system.